[ PATCH] execve: must clear current->clear_child_tid

Eric Dumazet
2009-07-31T10:01:03Z

While looking at Jens Rosenboom bug report about strange sys_futex call done
from a dying "ps" program, we found following problem.

clone() syscall has special support for TID of created threads.
This support includes two features.

One (CLONE_CHILD_SETTID) is to set an integer into user memory
with the TID value.

One (CLONE_CHILD_CLEARTID) is to clear this same integer once
the created thread dies.

The integer location is a user provided pointer, provided at clone()
time.

kernel keeps this pointer value into current->clear_child_tid.

At execve() time, we should make sure kernel doesnt keep
this user provided pointer, as full user memory is replaced by a new one.

As glibc fork() actually uses clone() syscall with CLONE_CHILD_SETTID
and CLONE_CHILD_CLEARTID set, chances are high that we might corrupt
user memory in forked processes.

Following sequence could happen:

1) bash (or any program) starts a new process, by a fork() call
that glibc maps to a clone( ... CLONE_CHILD_SETTID
| CLONE_CHILD_CLEARTID ...) syscall

2) When new process starts, its current->clear_child_tid is set to a location
that has a meaning only in bash (or initial program) context (&THREAD_SELF->tid)

3) This new process does the execve() syscall to start a new program.
current->clear_child_tid is left unchanged (a non NULL value)

4) If this new program creates some threads, and initial thread exits,
kernel will attempt to clear the integer pointed by current->clear_child_tid
from mm_release() :

if (tsk->clear_child_tid
&& !(tsk->flags & PF_SIGNALED)
&& atomic_read(&mm->mm_users) > 1) {
u32 __user * tidptr = tsk->clear_child_tid;
tsk->clear_child_tid = NULL;

/*
* We don't check the error code - if userspace has
* not set up a proper pointer then tough luck.
*/
<< here >> put_user(0, tidptr);
sys_futex(tidptr, FUTEX_WAKE, 1, NULL, NULL, 0);
}

5) OR : if new program is not multi-threaded, but spied by /proc/pid users
(ps command for example), mm_users > 1, and the exiting program could
corrupt 4 bytes in a persistent memory area (shm or memory mapped file)

If current->clear_child_tid points to a writeable portion of memory
of the new program, kernel happily and silently corrupts 4 bytes of memory,
with unexpected effects.

Fix is straightforward and should not break any sane program.

Reported-by: Jens Rosenboom <jens@mcbone.net>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Jens Rosenboom <jens@mcbone.net>
---
fs/compat.c | 1 +
fs/exec.c | 1 +
2 files changed, 2 insertions(+)

diff --git a/fs/compat.c b/fs/compat.c
index 94502da..deb1049 100644
--- a/fs/compat.c
+++ b/fs/compat.c
@@ -1550,6 +1550,7 @@ int compat_do_execve(char * filename,
mutex_unlock(&current->cred_guard_mutex);
acct_update_integrals(current);
free_bprm(bprm);
+ current->clear_child_tid = NULL;
if (displaced)
put_files_struct(displaced);
return retval;
diff --git a/fs/exec.c b/fs/exec.c
index 4a8849e..e275652 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1343,6 +1343,7 @@ int do_execve(char * filename,
mutex_unlock(&current->cred_guard_mutex);
acct_update_integrals(current);
free_bprm(bprm);
+ current->clear_child_tid = NULL;
if (displaced)
put_files_struct(displaced);
return retval;